pkg/test: Fix a few analyzer warnings

lib/src/runner/browser/post_message_channel.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:convert';
 import 'dart:html';
 
 import 'package:stream_channel/stream_channel.dart';
 
 /// Constructs a [StreamChannel] wrapping `postMessage` communication with the
 /// host page.
 StreamChannel postMessageChannel() {
   var controller = new StreamChannelController(sync: true);
 
   window.onMessage.listen((message) {
@@ skipping 18 matching lines @@
 
   // Send a ready message once we're listening so the host knows it's safe to
   // start sending events.
   window.parent.postMessage({
     "href": window.location.href,
     "ready": true
   }, window.location.origin);
 
   return controller.foreign;
 }